Used often in industrial process measurements technology for ascertaining density of fluid flowing in a pipeline or stored in a container are measuring systems, in the case of which an oscillatably held, vibratory body, as part of a physical to electrical, measuring transducer, is brought into contact with the fluid to be measured, namely with a volume portion thereof. The vibratory body—contacted by fluid—is caused during operation to vibrate in such a manner, for example, actively by means of an electro-mechanical oscillation exciter acting on the vibratory body, that the vibratory body executes, at least partially, resonant oscillations, namely mechanical oscillations with a resonant frequency, which is dependent on the mechanical construction of the vibratory body, as well as also on the density of the fluid. The measuring transducer is, for such purpose, most often applied in a container wall of the container, for example, a tank, holding the fluid or in the course of a line, for example, a pipeline, through which the fluid is moving, and is equipped also to register vibrations of the vibratory body and to produce at least one oscillation measurement signal, which has at least one signal component with a signal frequency corresponding to the resonant frequency and, consequently, dependent on the density of the fluid. Examples of such measuring transducers, respectively measuring systems, formed by means of one or more vibratory bodies and thus suitable for measuring density, are described in, among others, EP-A 564 682, EP-A 919 793, US-A 2007/0028663, US-A 200810127745, US-A 2010/0083752, US-A 2010/0236323, US-A 2011/0219872, U.S. Pat. No. 4,524,610, U.S. Pat. No. 4,801,897, U.S. Pat. No. 5,027,662, U.S. Pat. No. 5,054,326, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,965,824, US-A 60 73495, U.S. Pat. No. 6,138,507, U.S. Pat. No. 6,148,665, U.S. Pat. No. 6,044,694, U.S. Pat. No. 6,389,891, U.S. Pat. No. 6,651,513, U.S. Pat. No. 6,688,176, U.S. Pat. No. 6,711,942, U.S. Pat. No. 6,845,663, U.S. Pat. No. 6,912,904, U.S. Pat. No. 6,938,475, U.S. Pat. No. 7,040,179, U.S. Pat. No. 7,102,528, U.S. Pat. No. 7,272,525, U.S. Pat. No. 7,549,319, U.S. Pat. No. 7,681,445, U.S. Pat. No. 7,874,199, WO-A 00/19175, WO-A 01/02816, WO-A 01/29519, WO-A 88/02853, WO-A 93/01473, WO-A 93/19348, WO-A 93/21505, WO-A 94/21999, WO-A 95/03528, WO-A 95/16897, WO-A 95/29385 or WO-A 98/02725. In accordance therewith, the vibratory body can be e.g. a measuring tube inserted into the course of the line carrying the fluid, thus a measuring tube through which the fluid is flowing—for instance, the measuring tube of a measuring transducer of a measuring system in the form of a purely density measuring device for flowing fluids, in the form of a Coriolis, mass flow/-density measuring device and/or in the form of a density-/viscosity measuring device—or, however, e.g. also a vibratory body formed by means of an oscillating cylinder extending into the fluid—located in a line or in a container—and formed, in given cases, in rod- or paddle shape and/or internally hollow, consequently provided e.g. also by a vibronic fill level limit switch measuring also density, supplementally to a limit of fill-level.
The measuring transducer is, furthermore, connected with an electronics of the measuring system serving for evaluation of the at least one oscillation measurement signal and for generating corresponding density, measured values representing the density. In the case of modern measuring systems of the type being discussed, such electronics are, as described in, among others, also in U.S. Pat. No. 6,311,136 or U.S. Pat. No. 6,073,495, most often implemented by means of one or more microprocessors formed, in given cases, also as digital signal processors (DSP). Besides evaluation of the at least one oscillation measurement signal delivered by the measuring transducer and representing oscillations of its vibratory body, the electronics serves also to generate at least one driver signal, for example, an harmonic and/or clocked, driver signal, for an electro-mechanical oscillation exciter acting on the vibratory body and serving for actively exciting said oscillations, for example, an electro-mechanical oscillation exciter having an exciter coil interacting with a permanent magnet affixed on the vibratory body or a piezoelement affixed on the vibratory body, wherein the driver signal has a signal component with a signal frequency matched to the resonant frequency of the vibratory body. The signal component, respectively the driver signal, can, for example, also be controlled as regards its electrical current level and/or voltage level.
In the case of measuring systems of the type being discussed, the electronics is most often accommodated within at least one, comparatively robust, especially impact-, pressure-, and/or weather resistant, electronics housing. The electronics housing can be arranged, for example, remotely from the measuring transducer and connected therewith only via a flexible cable; it can, however, also be arranged, as shown e.g. also in the initially mentioned U.S. Pat. No. 5,796,011, directly on the measuring transducer or on a measuring transducer housing separately housing the measuring transducer, and thus, also, its vibratory body. Moreover, however, as shown in, among others, WO-A 01/29519, it is also quite usual, in given cases, to use modularly formed electronics accommodated in two or more separate housing modules for forming measuring systems of the type being discussed.
In the case of measuring systems of the type being discussed, the electronics is usually electrically connected via corresponding electrical lines to a superordinated electronic data processing system most often arranged spatially removed from the respective device. Most often, the electronic data processing system is also spatially distributed. Measured values produced by the respective measuring system are forwarded near in time by means of a measured value signal correspondingly carrying the measured values. Measuring systems of the type being discussed are additionally usually connected with one another by means of a data transmission network provided within the superordinated data processing system and/or with corresponding electronic process controls, for example, on-site programmable logic controllers or process control computers installed in a remote control room, where the measured values produced by means of the respective measuring system and digitized and correspondingly coded in suitable manner are forwarded. By means of such process control computers, the transmitted measured values can be further processed and visualized as corresponding measurement results e.g. on monitors and/or converted into control signals for other field devices embodied as actuating devices, such as e.g. magnetically operated valves, electric-motors, etc. Since modern measuring arrangements can most often also be monitored and, in given cases, controlled and/or configured directly from such control computers, operating data intended for the measuring system are equally dispatched in corresponding manner via the aforementioned data transmission networks, which are most often hybrid as regards the transmission physics and/or the transmission logic. Accordingly, the data processing system serves usually also, to condition the measured value signal delivered by the measuring system in accordance with the requirements of downstream data transmission networks, for example, suitably to digitize the measured value signal and, in given cases, to convert such into a corresponding telegram, and/or to evaluate such on-site. For this purpose, there are provided in the data processing systems, electrically coupled with the respective connecting lines, evaluating circuits, which pre- and/or further-process as well as, in case required, suitably convert, the measured values received from the respective measuring system. Serving for data transmission in such industrial data processing systems are at least sectional, especially serial, fieldbusses, such as e.g., FOUNDATION FIELDBUS, RACKBUS-RS 485, PROFIBUS, etc., or, for example, also networks based on the ETHERNET standard, as well as the corresponding, most often comprehensively standardized, transmission protocols. Alternatively or supplementally, in the case of modern measuring systems of the type being discussed, measured values can also be transmitted wirelessly per radio to the respective data processing system.
Besides the evaluating circuits required for processing and converting the measured values delivered from the respectively connected measuring system, such superordinated data processing systems have most often also electrical supply circuits serving for supplying the connected measuring- and/or switching devices with electrical energy. Such electrical supply circuits provide a supply voltage for the respective electronics and drive the electrical currents flowing through electrical lines connected thereto as well as through the respective electronics. In given cases, such voltage is fed directly from the connected fieldbus. A supply circuit can, in such case, be associated with, for example, exactly one measuring system, respectively a corresponding electronics, and can be accommodated, together with the evaluating circuit associated with the respective measuring system —, for example, united into a corresponding fieldbus adapter—in a shared electronics housing, e.g. in the form of a hatrail module. It is, however, also quite usual to accommodate supply circuits and evaluating circuits, in each case, in separate electronics housings, in given cases, spatially remote from one another and to wire them together via external lines.
In the case of measuring systems for density measurement, wherein the measuring system operates by means of a vibratory body, such as disclosed in, among others, the initially mentioned WO-A 88/02853, WO-A 98102725, WO-A 94/21999, in the case of ascertaining the density ρ based on the resonant oscillations of the vibratory body, or its resonant frequency fr, the temperature ϑ10 of the vibratory body, thus a temperature of the vibratory body dependent on a temperature of the fluid to be measured, consequently a variable temperature of the vibratory body, is to be take into consideration. For ascertaining such, a local temperature ϑsens of the vibratory body on a surface of the vibratory body facing away from the fluid, consequently a “dry” surface not contacted thereby, is registered by sensor, usually by means of a thereon adhered, platinum resistor of a resistance thermometer or by means of a thermocouple adhered on said surface, as well as a corresponding measuring circuit in the electronics. Such temperature is then correspondingly taken into consideration in ascertaining the density, for instance, according to the relationships, ϑsens˜ϑ10, fr2=f(ϑsens→ϑ10), or fr2=f(1/ρ). An additional improvement of the accuracy, with which the density can ultimately be measured, can be achieved in the case of measuring systems of the type being discussed, not least of all in the case of such having as vibratory body a measuring tube clamped on its two ends, in among other ways, by registering, as mentioned, among others, also in US-A 2011/0219872, furthermore, mechanical deformations of the vibratory body located in its static rest position, for instance, deformations as a result of a changing temperature of the vibratory body and/or as a result of forces acting on the vibratory body, or mechanical stresses within the vibratory body resulting therefrom and correspondingly taking such into consideration in calculating the density. Such mechanical deformations of the vibratory body can be registered, for example, by means of one or more strain sensors mechanically coupled with the vibratory body via its “dry” surface.
Further investigations on measuring systems of the type being discussed have, however, shown that, based on the measured temperature ϑ10 and resonant frequency fr, the density ρ of fluids can indeed be very exactly ascertained, namely directly with a relative measuring error of less than 0.2%, in cases where temperature remains constant over longer periods of time of several minutes or more. However, especially in the case of a change of the fluid in the line, the density measured for the “new” fluid can, first of all, deviate considerably from its actual density; this—even in the case of application of strain sensors—unluckily, at times, even in such a manner that, in the case of a fluid with a density actually reduced relative to the preceding fluid, first of all, a higher density than earlier is ascertained, respectively also, conversely, in spite of greater density for the “new” fluid, first of all, a lesser density is ascertained. Consequently, the measuring error for the density has, in comparison to its change, an opposite sign, respectively, the measuring system has, insofar, an all-pass characteristic.